It is frequently desirable to reinforce rubber articles, for example, tires, conveyor belts, power transmission belts, timing belts, hoses, and the like products, by incorporating therein steel reinforcing elements. Pneumatic vehicle tires are often reinforced with cords prepared from brass coated steel filaments. Such tire cords are frequently composed of high carbon steel or high carbon steel coated with a thin layer of brass. Such a tire cord can be a monofilament, but normally is prepared from several filaments which are stranded together. In most instances, depending upon the type of tire being reinforced, the strands of filaments are further cabled to form the tire cord.
In order for rubber articles which are reinforced with steel wire elements to function effectively it is imperative that good adhesion between the rubber and the steel cord be maintained. Steel is very prone to oxidation, which even in minor degrees is highly deleterious to rubber-metal adhesion. Thus, generally steel wire reinforcement elements are coated with brass in order to facilitate rubber-metal adhesion. It is generally agreed by those skilled in the art that adhesion of rubber to brass-plated steel wire is dependent upon a bond between the copper in the brass and sulfur in the rubber. When such brass coated steel reinforcing elements are present in the rubber composition during vulcanization, it is believed that bonds between the rubber and steel reinforcement gradually form due to a chemical reaction between the brass alloy and the rubber at the interface forming a bonding layer. At some point in the vulcanization procedure a maximum number of bonds is obtained. After this maximum is achieved the number of bonds present begins to decrease, probably by secondary reactions which decompose the layer. Subsequent to vulcanization and during the further lifetime of the steel reinforced article, these reactions continue at much lower rates by heat aging, for example, in a running tire, and this together with oxidative degradation of the rubber itself, contributes to further destruction of the bond. The temperature and duration of vulcanization must be well adapted to the metallic-coating in order to maximize rubber to metal adhesion.
As a general rule vulcanization temperatures in excess of 163.degree. C. (325.degree. F.) are detrimental to good rubber to brass adhesion. Thus, high temperature cure cycles have traditionally resulted in poor rubber to metal adhesion. For this reason high temperature cure cycles have not normally been employed to vulcanize rubber articles containing brass coated steel elements as reinforcements.
The amount of time needed to cure a rubber article decreases with increasing cure temperatures. Thus, by increasing cure temperatures, cure cycles can be reduced in duration. It is, therefore, possible to reduce the amount of time required in manufacturing vulcanized rubber articles by utilizing higher cure temperature.
By practicing this invention rubber articles which are provided with ternary iron-brass steel reinforcing elements can be cured at high temperatures while maintaining excellent rubber to metal adhesion. In many cases by practicing this invention, rubber to metal adhesion properties can be improved in metal reinforced articles which are cured using either a standard cure temperature or a high temperature cure. The practice of this invention generally results in both improved original adhesion and aged adhesion properties.